iTunes 8 introduced a new way to browse your music library, the grid view. In the grid view you can browse your libary by genre and iTunes displays a special image for each genre it knows.
The number of genres recognized by iTunes and receiving an appropriate genre artwork is limited though. Here the iTunes Genre Art Manager comes into play. It allows you to add images to genres for which iTunes does not bring its own art. Also you can modify existing genre artwork to your preference.